1. Field of the Inventive Concept(s)
The presently disclosed and/or claimed inventive concept(s) relates generally to hexagonal osmium boride, OsB2, and methods of producing the same. In one non-limiting embodiment, hexagonal OsB2 is produced by mechanochemical synthesis of osmium and boron in a high energy ball mill.
2. Background of the Inventive Concept(s)
Significant interest has developed over the last few decades regarding transition metal borides, such as, for example, OsB2, ReB2, RuB2, IrB1.1, and WB4, due to their unique properties, namely ultra-high compressibility and hardness. Hardness can be defined as a measure of a material's resistance to plastic indentation under an applied load. Specifically, hardness can be defined as the maximum indentation load applied to a material divided by the corresponding contact area, which can be measured using imaging techniques like SEM or optical microscopy. See Haines et al., “Synthesis and design of superhard materials.” Annu. Rev. Mater. Res., 31, 1-23, 2001, hereby expressly incorporated by reference herein in its entirety.
Chemical bonding between individual atoms determines the hardness of a material. The bond strength of a material correlates directly with both the elastic stiffness and mobility of dislocations (i.e., plastic deformation) of a material. Elastic stiffness is a measurement of the resistance of bonds to stretching and bending, wherein the resistivity to stretching is measured by the elastic bulk modulus and the resistivity to bending is measured by the shear modulus. To have a high elastic stiffness and likewise a high hardness value, both the bulk and shear moduli need to be maximized. Additionally, resistance to plastic deformation, as determined by the dislocation mobility of a material, should also be high in order for a material to have a high hardness value.
It has been known that ceramics like transition metal borides have a high hardness value, which enables them to be used in a similar manner as known “superhard” materials (i.e., materials having a hardness of at least 40 GPa, as measured by the Vickers hardness test), such as, for example, diamond and cubic boron nitride. Such properties have made transition metal borides and other ultra-hard ceramics an intriguing commodity specifically for industrial uses, for example, as coatings capable of reducing the amount of deformation and wear on various pieces of machinery including, for example but without limitation, cutting tools, blades, pistons, turbine blades, and other industrial instruments or products.
In particular, rhenium boride (ReB2) and osmium boride (OsB2) have received special attention due to their reportedly high valence-electron densities, which contribute to their ultra-high compressibility and hardness properties. Such properties are attributable to the presence of osmium or rhenium ions in their respective lattice structures as well as the high degree of bond covalency for the B—B bonds and Os—B or Re—B bonds in their respective OsB2 and ReB2 lattice structures.
Studies have determined, however, that current methods of incorporating boron into the lattice of osmium results in a lattice structure differing from the lattice structure formed when incorporating boron into the lattice of rhenium. Specifically, current methods of incorporating boron atoms into an osmium lattice cause the lattice to expand by about 10%, forming an orthorhombic Pmmm (No. 59, oP6 type) structure of OsB2 having lattice parameters of a=4.684 Å, b=2.872 Å, and c=4.076 Å (FIG. 1), while the incorporation of boron atoms into the interstitial tetrahedral site of rhenium causes only a 5% expansion of the original lattice, forming ReB2 with a hexagonal P63/mmc (NO. 194) structure having lattice parameters of a=2.9 Å and c=7.478 Å (FIG. 2). See Chung et al., “Synthesis of ultra-Incompressible superhard rhenium diboride at ambient pressure,” Science, 316 (2007), 436-439, and B. Aronsson, “The crystal structure of RuB2, OsB2, IrB1.35 and some general comments on chemistry of borides in the composition range MeB-MeB3,” Acta. Chem. Scand., 17 (1963) 2036, each of which is hereby expressly incorporated herein in its entirety. The smaller expansion of the ReB2 hexagonal lattice, as compared to the orthorhombic lattice of OsB2, results in the Re—Re metal bonds in ReB2 being shorter than the Os—Os bonds in OsB2, which leads to an increased bond strength and, in turn, an increased stiffness/hardness and an overall improvement of mechanical properties for ReB2, as compared to orthorhombic OsB2.
Studies have also determined that although orthorhombic OsB2 can withstand hydrostatic in situ compression up to around 32 to 36 GPa, orthorhombic OsB2 becomes unstable under tensile or shear deformation due to the orthorhombic shape. Specifically, around 20 GPa in tension and only 9.1 GPa in shear. In contrast, hexagonal ReB2 has a shear strength of about 34 GPa, much higher than the 9.1 GPa of OsB2. See Yang et al., “Is Osmium Diboride an Ultra-hard Material?,” J. Am. Chem. Soc., 130 (2008), 7200-7201, Cumberland et al., “Osmium diboride, an ultra-incompressible, hard material,” J. Am. Chem. Soc., 127 (2005) 7264-7265, and Gu et al., “Transition metal borides: Superhard versus ultra-incompressible,” Adv. Mater., 20 (2008) 3620-3626, each of which is hereby expressly incorporated herein in its entirety. The tendency for orthorhombic OsB2 to become unstable and deform under shear stress is due to the Os—Os metallic bonds within the orthorhombic structure being prone to deformation under applied shear stresses, which greatly reduces the resistance of the entire OsB2 structure against large shear deformation in certain easy-slip directions. As such, it was predicted that diviatoric stress could transform the crystalline structure of OsB2 into a different structure with relatively little force. See Chen et al., “Electronic and structural origin of ultra-incompressibility of 5d transition-metal diborides MB2 (M=W, Re, Os),” Phys. Rev. Lett., 100 (2008), 196403, and Ren et al., “Pressure induced structural phase transition of OsB2: First principle calculations,” J. Solid State Chem., 183 (2010) 915, each of which is hereby expressly incorporated herein in its entirety.
Chen et al. further predicted that three different crystalline structures could exist for OsB2, as illustrated in FIG. 3. The first crystalline structure being the above-described orthorhombic OsB2 structure (FIG. 3a). The second being a hexagonal structure similar to the above-described hexagonal ReB2 structure (FIG. 3b), herein referred to as the “Hex-I” structure, and the third being a hexagonal structure similar to that of the P6/mmm lattice structure of AlB2 (FIG. 3c), herein referred to as the “Hex-II” structure. Using local density calculations, Chen et al. also predicted, without specifying what type of stress would be necessary (i.e., uniaxial, hydrostatic, or shear), that it would only take 2.5 GPa of pressure to transform orthorhombic OsB2 into the hexagonal “Hex-I” form of OsB2—the lower compressibility of hexagonal OsB2 being one of the driving forces for the pressure-induced phase transition overcoming the relatively small (˜0.048 eV) potential energy difference between hexagonal OsB2 and orthorhombic OsB2. However, prior to the presently disclosed and/or claimed inventive concept(s), the only form of OsB2 that had been synthesized was the orthorhombic crystalline structure.
Additionally, it was also predicted that a stable hexagonal form of OsB2 would likely have higher bulk and shear moduli, i.e., an improved hardness, due to the shortened Os—Os bonds and less or none of the above-described structural weaknesses of the orthorhombic form of OsB2. In view of the foregoing, there is a need for both the hexagonal OsB2 itself, which prior to the presently disclosed and/or claimed inventive concept(s) was thought to only exist by way of mathematical calculations, and a method of producing the same.